Clearance Hole โ Bolt Fit & Drill Size
Calculate hole diameter for bolts: close, normal, or loose fit. Hole = bolt diameter + tolerance. Includes drill bit recommendations and countersink for flat-head screws.
Why This Construction Metric Matters
Why: Wrong clearance causes binding (too tight) or slop (too loose). Close for precision; loose for field assembly. Countersink for flush heads.
How: Hole = bolt diameter + tolerance. Close 0.15 mm; normal 0.4 mm; loose 0.9 mm. Countersink depth = (D/2) / tan(angle/2).
- โM6 normal fit โ 6.5 mm; use 6.5 or 1/4" drill.
- โClose fit: machined parts. Loose: rough construction.
- โCountersink dia โ 2ร head for flat-head screws.
- โISO metric; ANSI imperial. Match bolt standard.
๐ Sample Projects
M6 Close Fit
M6 bolt with close fit tolerance
1/4-20 Normal Fit
1/4" bolt with normal clearance
M10 with Countersink
M10 bolt with 90ยฐ countersink
5/16" Loose Fit
5/16" bolt with loose tolerance
Sheet Metal Application
M5 bolt for thin sheet metal
Wood Screw Pilot Hole
#8 wood screw clearance hole
M8 Precision Assembly
M8 bolt with close fit for precision
Custom Calculation
Enter your own specifications
M6
Diameter: 6.00 mm (0.2362")
๐ Results
Selected Fit
6.40 mm
Normal Fit
Close Fit
6.15 mm
Normal Fit
6.40 mm
Loose Fit
6.90 mm
๐ง Recommended Drill Bit
Size
1/4"
Metric
6.35 mm
Imperial
0.2500"
๐ Fit Type Comparison
Clearance Hole Diameters
Step-by-Step Calculation
=== Clearance Hole Calculation === Bolt Size: M6 (ISO) Bolt Diameter: 6.00 mm (0.2362") Fit Type: Normal Fit Tolerance: 0.40 mm (0.0157") === Clearance Hole Diameters === Close Fit: 6.15 mm (0.2421") Normal Fit: 6.40 mm (0.2519") Loose Fit: 6.90 mm (0.2716") Selected (Normal Fit): 6.40 mm === Recommended Drill Bit === Size: 1/4" (fractional) Diameter: 6.35 mm (0.2500") Difference: 0.05 mm
Planning estimates only. Verify with a licensed engineer or contractor before construction.
๐ Construction Industry Facts
Hole = Bolt diameter + tolerance. Close 0.15 mm; normal 0.4 mm.
โ ISO 273
Use next standard drill size; don't undersize.
โ ASME
Countersink depth = (D/2) / tan(angle/2).
โ Geometry
82ยฐ common for flat head; 90ยฐ standard.
โ Industry
๐ Key Takeaways
- โข Formula | Hole = Bolt Diameter + Tolerance
- โข Close 0.1โ0.2mm | Normal 0.3โ0.5mm | Loose 0.8โ1.0mm
- โข ISO metric | ANSI imperial. Standard drill bits recommended.
- โข Countersink | CS Dia โ 2ร head; depth = (D/2)/tan(angle/2)
Did You Know?
M6 bolt = 6mm diameter. Normal fit clearance โ 6.5mm. Use next standard drill size.
Source: ISO
Close fit for precision; loose for field assembly. Normal fit covers most applications.
Source: Tolerance
Drill bit sizes: fractional (1/4"), letter (AโZ), number (1โ80), metric (mm).
Source: Drill
82ยฐ countersink common for flat head screws. 90ยฐ and 100ยฐ also used.
Source: Countersink
Hole too small = binding. Hole too large = slop. Match fit to application.
Source: Assembly
ISO (metric) and ANSI (imperial) sizes. M3โM24 common; 1/4"โ1" imperial.
Source: Standards
What is a Clearance Hole Calculator?
A clearance hole calculator determines hole diameter for bolts and screws based on bolt size and fit tolerance. The hole must be larger than the bolt for easy insertion. Close, normal, and loose fits provide different assembly requirements. Includes drill bit recommendations and countersink calculations.
๐ฌ Key Concepts
Fit Type
Close = precision; Normal = general; Loose = field assembly.
Tolerance
Added to bolt diameter. Varies by fit.
Drill Bit
Use nearest standard size โฅ calculated hole.
Countersink
For flat head screws. Dia and depth by angle.
How to Calculate Clearance Holes
When to Use Fit Types
Close Fit
Minimal clearance for precise alignment
Tolerance: 0.15 mm
Precision assemblies, Machined parts, Tight tolerances
Normal Fit
Standard clearance for general applications
Tolerance: 0.4 mm
General construction, Most applications, Standard assemblies
Loose Fit
Generous clearance for easy assembly
Tolerance: 0.9 mm
Field assembly, Rough construction, Quick installation
Formulas Reference
Bolt Standards Reference
| Bolt | mm | in | Standard | Normal Clearance |
|---|---|---|---|---|
| 1" | 25.40 | 1.0000 | ANSI | 25.80 mm |
| M3 | 3.00 | 0.1181 | ISO | 3.40 mm |
| M4 | 4.00 | 0.1575 | ISO | 4.40 mm |
| M5 | 5.00 | 0.1969 | ISO | 5.40 mm |
| M6 | 6.00 | 0.2362 | ISO | 6.40 mm |
| M8 | 8.00 | 0.3150 | ISO | 8.40 mm |
| M10 | 10.00 | 0.3937 | ISO | 10.40 mm |
| M12 | 12.00 | 0.4724 | ISO | 12.40 mm |
| M14 | 14.00 | 0.5512 | ISO | 14.40 mm |
| M16 | 16.00 | 0.6299 | ISO | 16.40 mm |
Important Considerations
โ ๏ธ Avoid
- โข Hole too small (binding)
- โข Wrong fit for application
- โข Mismatched units (mm vs in)
โ Best Practices
- โข Use standard drill sizes
- โข Normal fit for most applications
- โข Check material thickness for countersink
โ ๏ธ Disclaimer: Clearance values are guidelines. Consult ISO 273, ANSI/ASME standards, or engineering specs for critical applications.
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